Distance estimation based on image contrast

ABSTRACT

The determination of an apparent distance between an object and a surface is disclosed. One disclosed embodiment comprises illuminating the object with a spatially inhomogeneous irradiance from the surface, an intensity variation in the spatially inhomogeneous irradiance in a plane parallel to the surface responsive to a distance between the plane and the surface. Next, an image of the object is acquired while the object is illuminated by the spatially inhomogeneous irradiance, and the apparent distance of the object from the surface is determined based on a brightness contrast in the image of the object due to the spatially inhomogeneous irradiance.

BACKGROUND

Computer systems receive various forms of input and generate variousforms of output, of which sound, text, and graphics are examples. Textand graphics may be displayed on a display surface, such as a monitor.Some computing devices may integrate input and output functionalitywithin a common physical structure—for instance, by imparting inputcapability to an interactive display surface. Touch-sensitive displayscreens are examples of this approach.

Touch-sensitive devices may utilize any of a number of possible touchsensing mechanisms, including, but not limited to, optical, resistive,and capacitive mechanisms. In any of these forms, a touch-sensitivedevice may allow the detection and utilization of input that is based ona position of one or more physical objects on a touch-sensitive displaysurface. For example, the acts of placing an object on a surface,lifting it off a surface, and moving the object from one location on asurface to another each may be a form of input.

SUMMARY

In one embodiment, a method for determining an apparent distance betweenan object and a surface is provided. The method comprises illuminatingthe object with a spatially inhomogeneous irradiance from the surface,an intensity variation in the spatially inhomogeneous irradiance in aplane parallel to the surface responsive to a distance between the planeand the surface. The method further comprises acquiring an image of theobject while the object is illuminated by the spatially inhomogeneousirradiance, and, determining the apparent distance based on a brightnesscontrast in the image of the object, the brightness contrast determinedfrom a difference in brightness in the image of the object due to thespatially inhomogeneous irradiance.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe Detailed Description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the Detailed Description. Further,the claimed subject matter is not limited to implementations that solveany disadvantages noted above or in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a disposition of an embodiment of an object locatedon an embodiment of an optical touch-sensitive surface, in accordancewith the present disclosure.

FIG. 2 shows three hypothetical functions plotted on a common coordinatesystem, each function representing a reflected light intensity from animage of the same object at different distances from an opticaltouch-sensitive surface, in accordance with the present disclosure.

FIG. 3 represents an image of an embodiment of an object at two exampledistances from an optical touch-sensitive surface, in accordance withthe present disclosure.

FIG. 4 illustrates a process flow of an embodiment of a method fordetermining an apparent distance between an object and a surface, inaccordance with one embodiment of the present disclosure.

FIG. 5 illustrates a process flow of an embodiment of a method fordetermining an apparent distance between an object and a surface, inaccordance with another embodiment of the present disclosure.

FIG. 6 shows in cross-section an embodiment of a system for determiningan apparent distance between an object and a surface, in accordance withthe present disclosure.

FIG. 7 shows a sectional view of another embodiment of an illuminator.

DETAILED DESCRIPTION

FIG. 1 illustrates a disposition of an object disposed over a surface inone example embodiment. In particular, the drawing shows substrate 102having surface 104. The substrate 102 may be a table top, a bench top, acounter top, or a work space of arbitrary size and function. In theillustrated embodiment, surface 104 is shown to be horizontal, but inother embodiments it may have any other suitable configuration and/ororientation.

In FIG. 1, substrate 102 is supported by pedestal 106, which elevatessurface 104 above another surface: a floor, a larger table top, etc.Console 108 comprises the substrate and the pedestal, as well as anyinternal structure on or within the substrate and/or the pedestal.Console 108 may include electronics, e.g., display and imagingelectronics, and associated optics. In one, non-limiting example,console 108 may be a surface computing device, and surface 104 may be adisplay surface of the surface computing device.

In this and other contemplated embodiments, console 108 may beconfigured to generate an output (a display output at surface 104, forexample) and to accept one or more forms of input, (a touch inputassociated with surface 104, for example). Some forms of input may bebased on a disposition of an object relative to the surface. Thus, FIG.1 shows object 110, which is illustrated as an arbitrary object. Object110 may be a stylus, a pen, a phone, a personal digital assistant, abook, a beverage glass, or virtually any object that can be placed on,lifted off, or moved upon the surface.

In some embodiments, a determination of whether or not object 110 is indirect contact with surface 104 may be a form of input to console 108.In other embodiments, a height (H) of the object above the surface maybe a form of input to the console. Thus, the balance of this disclosureis directed to systems and methods for determining an apparent distancebetween the object and the surface and for determining whether or notthe object is in direct contact with the surface.

When an object is illuminated with a spatially inhomogeneous irradiance,an intensity variation in the irradiance may effect a correspondingbrightness contrast in a detectable image of the object. Further, suchintensity variation may become larger or smaller with increasingdistance from the origin of the irradiance. As a result, thecorresponding brightness contrast in the image may depend on thedistance between the object and the origin of the spatiallyinhomogeneous irradiance. For example, if the spatially inhomogeneousirradiance is divergent, the intensity variation may diminish withincreasing distance from the origin, such that the correspondingbrightness contrast in the image decreases as the object moves fartherfrom the origin. In particular, when the object is relatively close tothe origin, the image may exhibit a strong, sharp brightness contrast;when the object is relatively far from the origin, the image may exhibita weaker, blurrier brightness contrast, the blurriness increasing withincreasing distance between the origin and the object. It will beunderstood that the term “brightness contrast,” as used herein, refersto a difference in brightness within an image of an illuminated portionof the object arising from a spatially inhomogeneous illumination of theobject.

FIG. 2 shows three hypothetical functions plotted on a common coordinatesystem. Each function represents a reflected light intensity from animage of the same object. Each function is plotted against distanceacross an example spatially inhomogeneous irradiance field. In oneembodiment, the irradiance field may be disposed along a surface, suchas surface 104 of FIG. 1, where the horizontal axis in FIG. 2 representsa horizontal distance, for example, from arbitrary location A toarbitrary location B shown in FIG. 1.

The origin of the spatially inhomogeneous irradiance giving rise to theillustrated plots is represented at horizontal axis 202, where the clearbars indicate regions of lambertian (i.e., isotropic, omnidirectional)irradiance and the dark bars indicate regions of little or noirradiance. The lambertian irradiance may be in the form of “point”sources (i.e. point-like or pseudo-point sources) arranged in a lineand/or line sources spanning a plane, e.g. the plane of surface 104. Itwill be understood that the number of and spacing of light sourcesillustrated in FIG. 2 is shown for the purpose of example, and that anysuitable number and spacing of light sources may be used.

First plot 204 represents a reflected light intensity in the image whenthe object is close to the origin of the spatially inhomogeneousirradiance, for example, in contact with the surface from which theirradiance emanates. The first plot exhibits relatively intense maxima,where the reflected light intensity is great, and relatively deepminima, where the reflected light intensity is low. The alternatinglight and dark regions in the image result from and correspond to theintensity variations at the origin of the spatially inhomogeneousirradiance.

Some embodiments may be configured so that the reflected light intensityapproaches zero at the minima (I₀˜0), while other embodiments may beconfigured so that the light intensity is significant at the minima.Factors that may affect the magnitude of the light intensity at theminima include, but are not limited to whether images of the object areacquired using a single or dual-pass illumination mode (vide infra).Briefly, deeper minima may be appropriate in embodiments where aseparate, substantially homogeneous source of illumination is used inaddition to the spatially inhomogeneous irradiance to locate objects,such an illumination mode being particularly useful for reading a valueof a tag, bar code, or other detailed object (where zero intensity mayincrease the difficulty of reading such an object).

Continuing with FIG. 2, second plot 206 represents a reflected lightintensity from the image when the object is farther from the origin ofthe spatially inhomogeneous irradiance than in the first plot. Themaxima of the second plot are less intense and the minima are shallowerthan in the first plot. While the intensity variations at the origin ofthe irradiance are still represented in the reflected image, they areblurrier and less clearly defined.

Third plot 208 represents a reflected light intensity from the imagewhen the object is yet farther from the origin of the spatiallyinhomogeneous irradiance than in the second plot. The minima and maximain the third plot are strongly attenuated compared to the otherfunctions, and the reflected intensity over the entire irradiance fieldstays relatively close to its average value, the intensity variations atthe origin of the irradiance causing relatively small deviations fromthe average value.

A spatially inhomogeneous irradiance such as that illustrated in FIG. 2may emanate from a surface, such as surface 104 of FIG. 1, to allow adetermination of a distance of an object from the surface. In theseembodiments, the origin of the spatially inhomogeneous irradiance maylocated close to or at the surface. When such irradiance emanates from asurface, the brightness contrast in an image of an object may differmarkedly depending on whether the object is in contact with the surfaceor whether it is disposed above the surface. FIG. 3 illustrates thiseffect in one, non-limiting example.

Specifically, FIG. 3 represents images of the bottom of object 110 intwo example dispositions. First image 300 is an image of the bottom ofthe object when the object is in contact with surface 104. The firstimage presents relatively high brightness contrast corresponding to anintensity variation of the spatially inhomogeneous irradiance. Secondimage 350 is an image of the bottom of the object when the object isdisposed above surface 104 by a height H. The second image also presentsa brightness contrast corresponding to the intensity variation of thespatially inhomogeneous irradiance, but the brightness contrast in thesecond image is reduced relative to that of the first image.

The change in brightness contrast of an image of an object caused by theheight of an object above a surface may be utilized to compute anapparent distance between an object and a surface, and/or to determinewhether or not the object is in contact with the surface. Prior todiscussing embodiments of methods for determining a distance between anobject and a surface, it will be understood that such methods may beperformed by computer-executable instructions or code, such as programs,stored in computer-readable storage media and executed by a processor toimplement the methods. Generally, programs include routines, objects,components, data structures, and the like that perform particular tasksor implement particular abstract data types. The term “program” as usedherein may connote a single program or multiple programs acting inconcert, and may be used to denote applications, services, or any othertype or class of program.

FIG. 4 illustrates a process flow for determining an apparent distancebetween an object and a surface in one, non-limiting example. Inparticular, FIG. 4 illustrates an embodiment of a singleillumination-phase process 400. The term “single illumination-phase” asused herein refers to a process wherein a spatially inhomogeneousirradiance is used to provide illumination for each acquired image. Incontrast, “dual-phase illumination”, as described below, refers to aprocess wherein a spatially inhomogeneous irradiance and another,substantially homogeneous irradiance are used to provide illuminationfor acquired images.

Single illumination-phase process 400 begins at 402, where an object isilluminated with a spatially inhomogeneous irradiance from a surface, anintensity variation in the spatially inhomogeneous irradiance in a planeparallel to the surface responsive to a distance between the plane andthe surface. In some embodiments, the spatially inhomogeneous irradiancemay comprise one or more infrared wavelengths.

The spatially inhomogeneous irradiance may be produced in any suitablemanner. For example, as described below, the spatially inhomogeneousirradiance may result from one or more spatially separated, radiantfeatures. Further, in some embodiments, the spatially inhomogeneousirradiance may comprise a regular pattern of irradiance, the regularpattern including radiant points, radiant lines, etc., while in otherembodiments, the spatially inhomogeneous irradiance may have a random,pseudo-random, or otherwise non-periodic structure. Non-periodicstructure in the irradiance may help to avoid aliasing issues that couldarise when a topology or coloration of the object, such as a bar code,is periodic or otherwise similar to the illumination pattern.

At 404, a first image of the object is acquired while the object isilluminated by the spatially inhomogeneous irradiance. In someembodiments, acquiring the first image comprises acquiring an image ofthe entire surface or a substantial portion of the surface that includesthe object, and then locating the object in the image by detecting anedge region of the object within the acquired image.

Next, at 406, a blurred, second image is derived from the first image.The blurred, second image is configured to have a generally evenintensity across the image, and may be computed using any suitableblurring method, including but not limited to averaging methods, Fourierfiltering methods, etc. Then, at 408, the first image is normalizedrelative to the second image. In one embodiment, normalizing the firstimage relative to the second image may include dividing a brightnessvalue of each pixel in the first image by that of the correspondingpixel in the second image.

Single illumination-phase process 400 next comprises, at 410,determining an apparent distance between the object and the surfacebased on a brightness contrast in the first image, where the brightnesscontrast corresponds to the intensity variation within the image of theobject. The apparent distance may be determined to decrease as thebrightness contrast increases. Likewise, the apparent distance may bedetermined to increase as the brightness contrast decreases. Thebrightness contrast within the image may be determined in any suitablemanner. For example, determining the contrast may include locatingmaximum and minimum contrast values within the image of the object.Computation at 410 may further include evaluation of afunction—analytical, numerical, and/or parametric—that maps the selectedbrightness contrast to the apparent distance.

In other examples, determining whether an object is on the surface maycomprise correlating an acquired image of the object with a referenceimage, e.g., a stored image of a uniformly colored object in contactwith the surface. Positive correlation between the acquired image andthe reference image across regions of the acquired image may indicatethat the object is on the surface. Such regions may encompass entireobjects comprising a large number of pixels or small target regionscomprising small numbers of pixels. Further, a level of correlationbetween acquired and reference images may provide a measure of theapparent distance of the object (or a region of the object) from thesurface.

In still other examples, estimating the apparent distance between theobject and the surface may comprise correlating the acquired image witha plurality of reference images, e.g. stored images of a white objectdisposed at various distances from the surface.

FIG. 5 illustrates an embodiment of a dual illumination-phase process500. To avoid repetition, events in dual illumination-phase process 500that correspond to those in single illumination-phase process 400 willbe identified with a minimum of description. It should be understood,however, that corresponding steps in the single and dualillumination-phase processes may be substantially the same, or may bedifferent.

The illustrated dual illumination-phase process begins at 502, where anobject is illuminated with a spatially inhomogeneous irradiance from thesurface. At 504, a first image of the object is acquired while theobject is illuminated by the spatially inhomogeneous irradiance. At 506,the object next is illuminated with a substantially homogeneousirradiance from the surface. Then, at 508, a second image of the objectis acquired while the object is illuminated by the substantiallyhomogeneous irradiance. At 510, the first image is normalized relativeto the second image. Thus, in dual illumination-phase process 500, theimage used for normalization is an authentic image of the objectacquired under substantially homogeneous irradiance. Normalizingrelative to an authentic image may offer certain advantages overnormalizing relative to a mathematically blurred image derived from thefirst image, particularly when the object has an inhomogeneous topologyand/or coloration. Then, at 512, an apparent distance between the objectand the surface is computed based on a brightness contrast in the imageof the object, where the brightness contrast corresponds to an intensityvariation in the spatially inhomogeneous irradiance.

In embodiments in which determining an apparent distance between theobject and the surface comprises determining whether the object is or isnot in contact with the surface, the method may include (at 408 ofsingle-illumination phase process 400 and/or 512 of dual-illuminationphase process 500, for example) determining that the object is incontact with the surface if a brightness contrast in the image is abovea threshold, and, determining that the object is not in contact with thesurface if a brightness contrast in the image is below the threshold. Inother embodiments, an estimated actual distance between the object andthe surface may be determined.

FIG. 6 shows a sectional view of an embodiment of a system fordetermining an apparent distance between an object and a surface. As anaid to illustration, FIG. 6 also shows object 110 in two exampledispositions. A first illuminator 602 is shown that comprises one ormore light sources (e.g. 604 and 605) and first light guide 606. Thefirst light guide includes a plurality of concave structural features(e.g. concave feature 607) configured to cause light from the one ormore light sources to diverge from surface 104, forming at least part ofthe spatially inhomogeneous irradiance. In some embodiments, the one ormore light sources comprise one or more infrared light-emitting diodes(IR-LEDs). First illuminator 602 is configured to output spatiallyinhomogeneous light, as described above.

It will be understood that the dimensions of the components shown inFIG. 6, including but not limited to the illustrated relativethicknesses of the depicted light guides, as well as in other figuresdescribed below, may not be drawn to scale; some dimensions may beexaggerated to provide clarity or emphasis, or for other illustrativepurposes, and are not intended to be limiting in any manner.

In FIG. 6, surface 104 is an upper surface of first illuminator 602. Thefirst illuminator 602 is configured to provide a spatially inhomogeneousirradiance from the surface, an intensity variation in the spatiallyinhomogeneous irradiance in a plane parallel to the surface responsiveto a distance between the plane and the surface. As noted previously,surface 104 may be a display surface configured to transmit a displayimage. In the illustrated embodiment, the display image is provided tothe surface via liquid crystal display 608. Liquid crystal display 608is transparent to one or more wavelengths of light that are emitted byone or more light sources (e.g., 604 and 605) and reflected from object110. In that manner, an image of the object may be acquired while theobject is illuminated by first illuminator 602 via a camera disposed onthe opposite side of the liquid crystal display. In other embodiments, acamera may be disposed at any other suitable location.

Accordingly, FIG. 6 shows object 110 and camera 610 disposed on oppositesides of surface 104. The term “camera” is used herein to describe anydevice configured to detect and/or acquire at least a partial an imageof an object. In one embodiment, the camera may detect infrared light ofthe wavelength emitted by the first illuminator 602. Camera 610 may beconfigured to acquire an image of the object while the object isilluminated by the spatially inhomogeneous irradiance.

FIG. 6 shows processor 612, configured to execute instructions, andmemory 614. In this embodiment, the memory comprises instructionsexecutable by the processor to enable a computation of the apparentdistance based on a brightness contrast in the image of the object, thebrightness contrast corresponding to an intensity variation of thespatially inhomogeneous irradiance.

FIG. 6 shows second illuminator 616 configured to provide asubstantially homogeneous irradiance to the object, wherein camera 610is further configured to acquire a second image of the object while theobject is illuminated by the substantially homogeneous irradiance, andwherein processor 612 is further configured to compute the apparentdistance based on a brightness contrast in the first image normalizedrelative to the second image. For the reasons cited above with regard toliquid crystal display 608, it may be advantageous for the liquidcrystal display 608 to be substantially transparent to one or morewavelengths emitted by the second illuminator 616 and reflected from theobject. In some embodiments, the second illuminator may comprise one ormore IR-LEDs, and/or any other suitable infrared light-emitting source

In other embodiments, shown by example in FIG. 7, the first illuminatorcomprises a light guide, wherein the light guide includes a plurality ofstructural features configured to cause light from the one or more lightsources to diverge from surface 104, forming at least part of thespatially inhomogeneous irradiance. The light guide of FIG. 7 is similarto that of FIG. 6, but includes a plurality of convex structuralfeatures, e.g., convex feature 702.

The embodiments of the light guides shown in the above-described figuresmay be formed in any suitable manner. For example, an embodiment of alight guide may be etched or machined from a monolithic substrate, maybe cast, extruded, molded, thermally formed under heat from athermoplastic material, or in any other suitable manner. It will furtherbe understood that other embodiments of light guides may have geometriesother than those depicted herein. For example, an embodiment light guidemay be substantially wedge-shaped from side to side to allow an image tobe projected from a microdisplay located at a side of the light guide,and/or may be configured to be supplied with light from one side only.

In each of the illustrated embodiments, an illuminator radiates lightfrom an origin in the form of one or more radiant points (where thestructural features are point feature) and/or one or more radiant lines(where the structural features are line features). As light is reflectedout of the depicted light guides by the structural features in the lightguides, the intensity of light within the light guides decreases from anedge of the light guides toward a center of the light guides. Therefore,the radiant points or lines decrease in emission intensity as a distancefrom the IR LEDs or other light source increases. To compensate for thiseffect, in the depicted embodiments, the one or more radiant pointsand/or one or more radiant lines are distributed such that a density ofmore brightly radiant points or lines (i.e. features that are relativelycloser to the LEDs) is lower than a density of less brightly radiantpoints or lines (i.e. features that are relatively farther from theLEDs). This configuration may be useful in providing more uniformillumination over the entire irradiance field than configurations inwhich the features are equally spaced.

The example control and estimation routines disclosed herein may be usedwith various system configurations. These routines may represent one ormore different processing strategies such as event-driven,interrupt-driven, multi-tasking, multi-threading, and the like. As such,the disclosed process steps (operations, functions, and/or acts) mayrepresent code to be programmed into computer readable storage medium ina control system. It should be understood that some of the process stepsdescribed and/or illustrated herein may in some embodiments be omittedwithout departing from the scope of this disclosure. Likewise, theindicated sequence of the process steps may not always be required toachieve the intended results, but is provided for ease of illustrationand description. One or more of the illustrated actions, functions, oroperations may be performed repeatedly, depending on the particularstrategy being used.

Finally, it should be understood that the systems and methods describedherein are exemplary in nature, and that these specific embodiments orexamples are not to be considered in a limiting sense, because numerousvariations are contemplated. Accordingly, the present disclosureincludes all novel and non-obvious combinations and sub-combinations ofthe various systems and methods disclosed herein, as well as any and allequivalents thereof.

1. A method for determining an apparent distance between an object and asurface, the method comprising: illuminating the object with a spatiallyinhomogeneous irradiance from the surface, an intensity variation in thespatially inhomogeneous irradiance in a plane parallel to the surfaceresponsive to a distance between the plane and the surface; acquiring animage of the object while the object is illuminated by the spatiallyinhomogeneous irradiance; and determining the apparent distance based ona brightness contrast in the image of the object due to the spatiallyinhomogeneous irradiance.
 2. The method of claim 1, wherein the apparentdistance is decreased as the brightness contrast increases, and theapparent distance is increased as the brightness contrast decreases. 3.The method of claim 1, wherein the image of the object is a first image,and further comprising illuminating the object with a substantiallyhomogeneous irradiance, acquiring a second image of the object while theobject is illuminated by the substantially homogeneous irradiance, andnormalizing the first image relative to the second image.
 4. The methodof claim 1, wherein the image of the object is a first image, andacquiring the first image comprises acquiring a second image thatincludes the object, and locating the first image in the second image bydetecting an edge region of the object in the second image.
 5. Themethod of claim 1, wherein the spatially inhomogeneous irradiancecomprises one or more infrared wavelengths.
 6. The method of claim 1,wherein the spatially inhomogeneous irradiance results from one or morespatially separated, radiant features.
 7. The method of claim 1, whereinthe spatially inhomogeneous irradiance comprises a regular pattern ofirradiance.
 8. The method of claim 7, wherein the regular pattern ofirradiance comprises one or more radiant points and/or one or moreradiant lines.
 9. The method of claim 8, wherein the one or more radiantpoints and/or one or more radiant lines are distributed such that adensity of more brightly radiant features is lower than a density ofless brightly radiant features.
 10. The method of claim 1, whereindetermining the apparent distance comprises determining whether theobject is in contact with the surface.
 11. The method of claim 1,wherein the image is a first image, and further comprising determining ablurred, second image derived from the first image, and normalizing thefirst image relative to the second image.
 12. A system for determiningan apparent distance between an object and a surface, the systemcomprising: an illuminator comprising a one or more light sources andconfigured to provide a spatially inhomogeneous irradiance from thesurface, an intensity variation in the spatially inhomogeneousirradiance in a plane parallel to the surface responsive to a distancebetween the plane and the surface; a camera configured to acquire animage of the object while the object is illuminated by the spatiallyinhomogeneous irradiance; a processor configured to executeinstructions; and memory comprising instructions executable by theprocessor to enable a computation of the apparent distance based on abrightness contrast in the image of the object due to the spatiallyinhomogeneous irradiance.
 13. The system of claim 12, wherein the objectand the camera are disposed on opposite sides of the surface.
 14. Thesystem of claim 12, wherein the one or more light sources comprises aninfrared light-emitting diode.
 15. The system of claim 12, wherein theilluminator further comprises a light guide, wherein the light guidecomprises a plurality of structural features configured to cause lightfrom the one or more light sources to diverge from the surface, formingat least part of the spatially inhomogeneous irradiance.
 16. The systemof claim 12, wherein the illuminator is a first illuminator, and furthercomprising a second illuminator configured to provide a substantiallyhomogeneous irradiance to the object, wherein the camera is furtherconfigured to acquire a second image of the object while the object isilluminated by the substantially homogeneous irradiance, and wherein theprocessor is further configured to compute the apparent distance basedon a brightness contrast in the first image normalized relative to thesecond image.
 17. A method for determining whether an object is incontact with a display surface, the method comprising: illuminating theobject with a spatially inhomogeneous irradiance from the displaysurface, an intensity variation in the spatially inhomogeneousirradiance in a plane parallel to the display surface responsive to adistance between the plane and the display surface; acquiring an imageof the object while the object is illuminated by the spatiallyinhomogeneous irradiance; and determining that the object is in contactwith the display surface if a brightness contrast in the image is abovea threshold; and determining that the object is not in contact with thedisplay surface if a brightness contrast in the image is below thethreshold.
 18. The method of claim 17, wherein the image of the objectis a first image, and further comprising illuminating the object with asubstantially homogeneous irradiance, acquiring a second image of theobject while the object is illuminated by the substantially homogeneousirradiance, and normalizing the first image relative to the secondimage.
 19. The method of claim 17, wherein the image is a first image,and further comprising determining a blurred, second image derived fromthe first image, and normalizing the first image relative to the secondimage.
 20. The method of claim 17, further comprising correlating atleast part of the image with a stored reference image, and determiningthat the object is in contact with the display surface if a correlationbetween the image and the stored reference image is above a threshold.